Electrophotographic production of colour photoconductive mosaic material

ABSTRACT

A process for the production of colour photoconductive material of the mosaic type, wherein areas of the material are sensitive only to selected regions of the visible spectrum which comprises: A. CHARGING THE SURFACE OF AN UNSENSITIZED PHOTOCONDUCTIVE MATERIAL, B. EXPOSING THE SURFACE OF THE MATERIAL TO A PATTERN OF LIGHT, C. TONING THE AREAS REMAINING CHARGED ON THE SURFACE OF THE PHOTOCONDUCTIVE MATERIAL BY A DYE SENSITIZER WHICH SENSITIZES THEM TO LIGHT OF ONE OF THE SELECTED REGIONS OF THE SPECTRUM, D. RECHARGING THE SURFACE OF THE PHOTOCONDUCTIVE MATERIAL, E. EXPOSING THE SURFACE OVERALL TO LIGHT WHICH COMPRISES ALL THE REGIONS OF THE VISIBLE SPECTRUM TO WHICH AREAS THEREOF HAVE BEEN SENSITIZED TO DESTROY THE CHARGE ON THE MATERIAL SELECTIVELY IN SUCH AREAS, F. EXPOSING THE SURFACE OF THE MATERIAL TO A DIFFERENT PATTERN OF LIGHT OF SUCH WAVELENGTH TO WHICH THE UNSENSITIZED PHOTOCONDUCTOR LAYER IS SENSITIVE, G. TONING THE AREAS REMAINING CHARGED ON THE SURFACE OF THE PHOTOCONDUCTOR BY MEANS OF A DYE SENSITIZER WHICH SENSITIZES THEM TO LIGHT OF A SELECTED REGION OF THE SPECTRUM TO WHICH NO AREA OF THE SURFACE OF THE MATERIAL IS AT PRESENT SENSITIVE AND REPEATING STEPS (D), (E) AND (G).

llnite States Patent [191 Hercock et al.

[111 3,754,908 1 Aug. 28, 1973 1 ELECTROPHOTOGRAPHIC PRODUCTION OF COLOUR PHOTOCONDUCTIVE MOSAIC MATERIAL [75] Inventors: Robert James Hercock; Simon Lindsay Scrutton, both of llford, England [73] Assignee: Iliord Limited, llford, Essex,

England [22] Filed: Apr. 13, 1972 [21] Appl. No.: 243,731

Related US. Application Data [63] Continuation-impart of Ser. No. 93,925, Dec. 1, 1970,

abandoned.

[30] Foreign Application Priority Data Dec. 8, 1969 Great Britain 59,824/69 [52] US. Cl 96/1.2, 96/1.6, 96/l.7, 1 17/175 [51] int. Cl G03g 13/22 [58] Field of Search 96/1.2, 1.3; 117/175 [56] References Cited UNITED STATES PATENTS 3,672,887 6/1972 Matsumoto et a1 96/1.2 3,329,590 7/1967 Renfrew 96/l.2 X

3,060,020 10/1962 Graig 96/1.2 3,060,021 10/1962 Graig 96/l.2 3,475,169 10/1969 Lange 96/l.2 X 3,615,392 10/1971 I-Ionjo 96/l.2 3,717,463 2/1973 Boch et al. 96/l.2 R

FOREIGN PATENTS OR APPLICATIONS 2,017,328 10/1970 Germany 96/1.2

Primary Examiner-Norman G. Torchin Assistant Examiner-John R. Miller, Jr. Attorney-John W. Malley, Paul N. Kokulis et al.

[57] ABSTRACT A process for the production of colour photoconductive material of the mosaic type, wherein areas of the material are sensitive only to selected regions of the visible spectrum which comprises:

a. charging the surface photoconductive material,

b. exposing the surface of the material to a pattern of light,

c. toning the areas remaining charged on the surface of the photoconductive material by a dye sensitizer which sensitizes them to light of one of the selected regions of the spectrum,

d. recharging the surface of the photoconductive material,

e. exposing the surface overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitized to destroy the charge on the material selectively in such areas,

f. exposing the surface of the material to a different pattern of light of such wavelength to which the unsensitized photoconductor layer is sensitive,

g. toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitizer which sensitizes them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive and repeating steps (d), (e) and (g).

of an unsensitized 12 Claims, 7 Drawing Figures PATENTEDMIB28 I875 SHEET-10F 3 FIG.3

ELECTROPHOTOGRAPHIC PRODUCTION OF COLOUR PHOTOCONDUCTIVE MOSAIC MATERIAL This application is a continuation-in-part application of our copending application Ser. No. 93,925 filed Dec. 1, 1970, now abandoned.

This invention to colour electrophotography.

Photo-conductive material usually comprises a layer of photo-conductive material coated onto a support base which has been rendered electro-conductive, or a layer of photo-conductive material coated onto a layer of electro-conductive material which is in its turn coated onto a support base. To produce an image on such photo-conductive material, the material is charged electrically, the charged material is then im- 1 age-wise exposed to light and in the areas where the light strikes the charge is dissipated. The image is developed usually by applying to the material a toner which comprises particles which adhere to the photoconductive layer in the areas which are still charged. it 2 is possible to produce colour photo-conductive material and in one proposed type of material the surface of the photo-conductor layer comprises a mosaic of areas each of which have been selectively spectrally sensitized to light in one of the three regions of the spectrum 2 red, green and blue. The term spectrum used herein refers to the visible spectrum unless otherwise identified.

In another type of photo-conductive process the material is not charged initially but when such material is image-wise exposed to light the photo-conductive material becomes electro-conductive in the areas where the light falls. These areas may then be toned by an electrolytic process to form an image. In a modification of this process colour photo-conductive prints may be obtained. For this process photo-conductive material is used which comprises a photo-conductive layer having on its surface discrete areas each of which have been selectively spectrally sensitized to one of three regions of the spectrum, red, blue and green, there being in each of these areas a chemical substance which is capable of forming a dye image which is complementary in colour to the colour of light absorbed by the area. After an imagewise exposure the material is electrolytically developed to stabilise the image and prevent chemical reaction taking place at a later stage in those areas which have been affected by light. The layer is then chemically developed by a coupling type developer to yield a tricolour dye image.

The term colour photo-conductive material of the mosaic type" as used hereinafter means photoconductive material which comprises a photoconductive layer which has been spectrally sensitized to more than one region of the spectrum and in such a 5 way that each discrete area of the surface is sensitive to light of only one region of the spectrum.

It is known that when a sensitising dye is applied to the top surface of an unsensitized photo-conductive layer, the layer itself becomes sensitive to light absorbed by the sensitising dye even though the lower regions of the layer of the photoconductive substance contain no dye. It is thus possible to produce a mosaic of areas each sensitive to a different region of the spectrum by successively applying sensitising dyes to a photo-conductive layer by use of a screen pattern, using conventional printing methods. It is also possible to produce such a mosaic pattern by electrophotographic means, for example in one such method a spectrally unsensitized photo conductive layer is charged and exposed to a screen pattern of light, the charge remaining on the surface is toned in a sensitising liquid which thus sensitizes those areas of the photoconductor. Further screen exposures, which expose different areas of the material, followed by the application of different sensitizers, enable the mosaic pattern of dye sensitization of discrete areas of the photoconductive layer to be builtup.

However both of the above described methods, for producing colour photo-conductive material of the mosaic type, suffer from the grave disadvantage that accurate registration of the screen images is necessary for the sensitizers to be properly applied onto the different areas of the photo-conductive surface so that the areas do not overlap.

According to the present invention there is provided a process for the production of colour photoconductive material of the mosaic type, wherein areas of the material are sensitive only to selected regions of the visible spectrum, from spectrally unsensitized photo-conductive material, which comprises either a photo-conductor layer coated on an electro-conductive layer which is in its turn coated on a support base or a photo-conductive layer coated on a support base which is or has been rendered electro-conductive, which comprises the following steps:

a. charging the surface of the photo-conductive material,

b. exposing the surface of the material to a pattern of light of such wavelength to which the unsensitized photo-conductor layer is sensitive,

c. toning the areas remaining charged on the surface of the photo-conductive material by means of a dye sensitizer which sensitizes them to light of one of the selected regions of the spectrum; followed by at least one further sequence of the steps d, e, f, and g which comprise,

d. recharging the surface of the photo-conductive material.

e. exposing the surface overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitized thereby to destroy the charge on the material selectively in such areas,

f. prior to, or at the same time as, or after this overall light exposure, exposing the surface of the material to another pattern of light of such wavelength to which the unsensitized photo-conductor layer is sensitive the said pattern being difi'erent from any pattern of light to which the material has been exposed previously,

g. toning the areas remaining charged on the surface of the photo-conductor by means of a dye sensitizer which sensitizes them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive, and then h. recharging the surface of the photo-conductive material,

i. exposing the surface of the material overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitized thereby to destroy the charge on the material selectively in such areas,

. toning the areas remaining charges on the surface of the photo-conductor by means of a dye sensitizer which sensitizes them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive.

If it is required that the resultant mosaic be sensitive to three selected regions of the spectrum then the process employed comprises carrying out the steps a, b, c, followed by one sequence of the steps d, e, f, and g, followed by the steps h, i, and j.

However if it is required that the resultant mosaic be sensitive to four selected regions of the spectrum then the process employed comprises carrying out the steps a, b, 0, followed by two sequences of the steps d, e, f, and g, followed by the steps h, i, and j.

For every additional region of the spectrum to which it is required that the resultant mosaic be sensitive a further sequence of the steps d, e, f and g must be employed.

It is to be understood that each pattern of light employed in each step f must be different and that each dye sensitiser employed in each step g must be different.

Preferably when the resultant mosaic is sensitive to only three selected regions of the spectrum the two patterns of light to which the photo-conductive material is exposed in a first and in a second exposure are such that the areas which are sensitive to only one of the three regions of the spectrum are equal.

Most preferably the three selected regions of the spectrum are red, blue and green.

Alternatively in the production of this mosaic mate- 1 rial if it is required that, for example, more of the photoconductive layer should be red sensitive, then the patterns of light are adjusted so that the red sensitive areas are bigger in area than the other areas.

One method of preparing colour photoconductive material of the mosaic type, wherein areas are each sensitive only to one of the three regions of the spectrum according to the present invention, is by use of two screens, one of which is a screen which comprises a series of light-opaque lines on a transparent background, the area occupied by the opaque lines occupying preferably a third of the area of the screen and eachopaque line being preferably of the same width.

The first exposure to light of a wavelength to which the unsensitised photoconductor is sensitive is carried out using a screen, preferably having light opaque lines thereon of equal width, the area of the light-opaque lines on this screen occupying half the area of the screen. The second exposure is carried out so that the lines on the second screen cross at an angle to the lines on the screen used during the first exposure. In this way equal areas of the surface of the photoconductor can be sensitised to the three main regions of the spectrum.

However in an alternative embodiment of this aspect of the present invention the second line screen has on it lines which are opaque to light of a wavelength to which the unsensitised photoconductor is sensitive but which transmit light of a wavelength to which areas of the photoconductor have been made sensitive in the first sensitising operation are sensitive. Thus in an exposure to white light through a screen of this type all the photoconductor layer is exposed to light in that region of the spectrum to which some areas thereof have just been sensitised. Usually the unsensitised photoconductor is sensitive only to light of the ultraviolet region of the spectrum. Thus if the screen comprised yellow lines on a clear background and the first sensitisation operation ,had made areas of the material sensitive to red-light, then one white light (containing ultraviolet light) exposure of the material through this screen would discharge both the red sensitive and the unsensitised areas of the material in the region of the material covered by the clear areas of the screen, but only the red sensitive areas in the region of the material covered by the yellow lines of the screen would be discharged.

It is preferred that in all the above embodiments of this aspect of the invention the lines on the screen in the second exposure cross the direction of the lines in the first exposure at right angles but it is to be understood that the same result may be obtained by arranging for the second exposure to be at an angle other than at right angles to the lines in the first screen exposure. Also a variation in the line width may be employed to produce unequal total areas in the mosaic. In fact in the second screen exposure the width of the lines on the screens may be chosen to produce any desired result in regard to the mosaic of areas produced.

The spacing of the line screens is determined by the resolution required. In general from to 300 lines per inch are required.

In another embodiment of the invention it is possible to produce photoconductive material the photoconductive surface of which comprises a mosaic of areas each of which have been spectrally sensitised to one of four regions of the spectrum.

As hereinbefore stated this process carrying out steps a, b and c, followed by two sequences of the steps (1, e, f and g, followed by steps h, i and j.

In this process it is preferred that the patterns of light to which the material is exposed in exposure b and the two exposures e are produced by exposing the material in each exposure to light of a wavelength'to which the material is sensitive through a screen which comprises a series of lines which are opaque to light of such wavelength on a light transparent background, the'lines in the second exposure crossing at an angle to the direction of the lines in the first exposure and the lines in the thirdexposure crossing at an angle to the direction of the lines both in the first exposure and in the second exposure.

Most preferably the lines of the screen employed in the said second exposure are at substantially 60 to the direction of the lines of the screen employed in the first exposure, and wherein in the said second exposure the lines of the screen employed in the third exposure are at substantially 60 both to the direction of the lines on the screen employed in the first exposure and to the lines on the screen employed in the second exposure.

It is to be understood that the line screens used in the exposures required in this embodiment of the invention can be transparent background screens having lines thereon which are opaque to all light or as previously described lines which are opaque only to light to which the unsensitised photoconductor is sensitive.

Preferably the width of the lines on the screens are so chosen that equal areas produced in the final mosaic pattern, for example the first screen has one-fourth of its area opaque, the second one-third and the third onehalf.

It is to be understood that if for any reason the total areas of the mosaic are required not to be equal the width of the lines on the screens may be so chosen to produce any desired result.

In all of the embodiments of the invention it is preferred that the photoconductor is a layer of zinc oxide in a binder. However instead of zinc oxide other organic or inorganic photoconductive substances may be used, e.g., polyvinyl carbazole.

A particular useful liquid which may be used to sensitise the areas of the photoconductor to one region of the spectrum is liquid sensitiser for colour sensitising the surface of a charged photoconductor layer which comprises a charged suspension of dye-sensitised photoconductive zinc oxide particles in an insulating carrier liquid. Optionally there is also present in the liquid a resin fixing agent for example an alkyd resin and also a charge controlling agent for example a metal soap as ferric stearate.

When the colour photoconductive material of the mosaic type is to be used in a process wherein a latent image is produced by an electrolytic process as hereinbefore described there is associated with each sensitiser a colour former which will produce a dye which is complementary in colour to the colour of the light absorbed by the sensitiser.

The accompanying drawings will serve to illustrate the invention.

FIGS. l-3 illustrate the production of colour photoconductive material of the mosaic type which comprises three sets of areas, each one of which is sensitive to one region of the spectrum. FIGS. 4-7 illustrate the production of colour photoconductive material of the mosaic type which comprises four sets of areas each one of which is sensitive to one region of the spectrum.

FIG. 1 shows photoconductive material after the first stage in the preparation of the mosaic material.

FIG. 2 shows the same photoconductive material after the second stage in the preparation of the mosaic material.

, FIG. 3 shows colour photoconductive material of the mosaic type which has been produced by the three stage process of the present invention.

The spectrally unsensitised photoconductive material used was prepared by coating a suspension of photoconductive zinc oxide (12 parts by weight) in a toluene solution of an alkyl resin (1 part) on to a conductive base paper the resin being present as the binder. The paper after drying was charged negatively to an 8KV corona charger.

The charged photoconductive material was exposed through a screen (A) for30 seconds at 12 inches from a 48 watt tungsten lamp operating at 2,850K. (The screen (A) used was a glass screen, a third of the area of which was covered by opaque black lines equally placed across the screen, there being 300 lines to the inch). After exposure the material was then sensitised by dipping it into a sensitising toner for 3 minutes, the composition of the toner being 1.0 g. of Patent Blue V dye-sensitised zinc oxide, 0.025 ferric stearate, 2.0 g. of an alkyd resin and 250 ml. odourless paraffin (1.03 Cps at C, at 166C 50 percent distilled over) as a carrier. The same odourless paraffin was used throughout the description hereinafter. After drying the material when highly magnified has lines on it as diagrammatically illustrated in FIG. 1, in which the areas 1 are the unsensitised areas and the areas 2 are the red sensitive areas. The material may appear white on casual inspection but if very closely examined is found to show light cyan lines (area 2).

After drying the photoconductive material was charged negatively to a 8KV corona charger and exposed l2 inches from a lOO watt tungsten lamp covered by an Ilford 204 red filter (Filter no 204 in Ilford Colour Filters Handbook published by Ilford Limited, England). This exposure discharges the material in the areas which have been sensitised with the Patent Blue V dyed zinc oxide, i.e., the red sensitive areas.

The material was then exposed through a screen (B) for 30 seconds at 12 inches from a 48 watt tungsten lamp operating at 2,850K, the lines' on screen (B) being normal to the lines of screen (A) during the previous exposure. Screen (B) consists of transparent glass on which are present parallel opaque lines covering half the area of the screen, there being 300 lines to the inch.

The material was then sensitised in a sensitising liquid similar to the liquid used above except that the zinc oxide had been sensitised by the use of Rose Bengal dye.

FIG. 2 shows diagrammatically a magnified portion of the photographic material after the completion of this stage.

In FIG. 2 the areas 1 are the unsensitised areas, the areas 2 are the red sensitised areas and the areas 3 are the green sensitised areas.

After drying, the material was again charged negatively to an 8KV corona charger and exposed 12 inches from a watt tungsten lamp covered by an Ilford yellow filter (Filter N0 110 in llford Colour Filters Handbook which absorbs all light below 500 nm and thus transmits all green and red light). The material was then sensitised in a similar liquid to those described above, except that the zinc oxide had been dyesensitised with Auramine O. This completes the mosaic pattern and FIG. 3 illustrates diagrammatically a magnified portion of the colour photoconductive material. In this figure, areas 2 are the red sensitised areas, areas 3 are the green sensitised areas and areas 4 are the blue sensitised areas. The sets of areas 2, 3 and 4 are substantially equal.

Colour photoconductive material of the mosaic type has thus been prepared by an photoconductive process without the need to provide careful registration of the screens used.

FIGS. 4 to 7 illustrate one method of producing colour photoconductive material which comprises four sets of areas each set of which is sensitive to one region only of the spectrum.

In FIG. 4 there is shown a magnified portion of photoconductive material after one screen exposure. In the Figure the areas 11 are the unsensitised material and the areas 12 are the areas which have been sensitised by the use of sensitiser A. The screen used in the exposure to produce this material comprises a series of lightopaque lines on a transparent background the areas occupied by the opaque lines being a quarter of the area of the screen, each line being of the same width.

In FIG. 5 the same photoconductive material is shown after a second screen exposure and sensitisation step has taken place. As in FIG. 4 areas 11 indicate the unsensitised material and areas 12 indicate the material which has been sensitised by sensitiser A, areas 13 indicate the material which has been sensitised by sensitiser B.

The screen used in the second exposure comprises a series of light-opaque lines on a transparent background with the same number of lines per inch as the first screen. In the second screen the opaque lines occupy one-third of the area of the screen and again each line is of the same width. In the second exposure the second screen is used at an angle of approximately 60 to the first screen in regard to the lines on each screen.

The material after the second exposure and sensitisation step comprises a series of parallelograms as indicated in FIG. 5. One method of ensuring that the final material has four sets of areas, each set being of equal area to the others, is to determine the angle at which the third screen is to be used and also the line spacing on the third screen from the dimensions of these parallelograms. If the long diagonal of the parallelograms is taken as one unit of length then the line spacing of the third screen should be two lines per unit of length. The third screen is used in a direction parallel to the short diagonal of the parallelograms.

FIG. 6 shows material after such a third exposure and after sensitisation with a third sensitiser C. In the Figure the unsensitised material is 11, the areas sensitised by sensitiser A are 12, the areas sensitised by sensitiser B are 13 and the areas sensitised by sensitiser C are 14.

Provided the third screen has the correct spacing and is applied at the correct angle there will be no register problems since whatever the position on the screen relative to the'lines already laid down on the material, after exposure and sensitisation by sensitiser C the areas so sensitised will occupy one half of the area of each parallelogram present on the material and thus one quarter of the total area of the surface.

FIG. 7 shows the material of FIGS. 6 after a final charging and sensitising step had taken place. Thus in the final material the areas 12 have been sensitised with sensitiser A, the areas 13 have been sensitised with sensitiser B, the areas 14 have been sensitised with sensitiser C and the areas 15 have been sensitised with sensitiser D.

It is to be understood that in the preparation of the materials shown in FIGS. 4 to 7 the same type of sensitising solutions are used as in the preparation of the material shown in FIGS. 1 to 3 and either before, at the same time as, or subsequent to each screen exposure, after the first screen exposure, the material is subjected to an overall exposure to light in the region of the spectrum or regions of the spectrum to which the areas, which have already been sensitised, are sensitive.

Thus photoconductive colour material has been produced which has a mosaic of equal sets of areas, each set of which is sensitive to one of the four regions of the spectrum. This material has been produced without any registration problems arising in the application of any of the screens during the requisite exposures.

EXAMPLE A. Preparation of Unsensitised Zinc Oxide Coated- Photoconductive Material Zinc oxide is ball-milled for 24 hours in the following formulation.

30 g zinc oxide 4.2 g alkyl resin ml toluene A further 15 ml toluene is added and the zinc oxide suspension dip coated at 25 g/m on electroconductive paper base. i B. Preparation of Sensitisers of the are dissolved in 40 ml methanol. 20g zinc oxide is stirred in, and the suspension evaporated to dryness.

After further drying for 24 hours at 50C the dye sensitised zinc oxide is ball-milled for 2 hours in the following formulation.

5 g dye sensitised zinc oxide 2 g soya modified alkyd resin 20 ml odourless paraffin The suspension is diluted to 500 ml with odourless Green Sensitiser The green sensitiser is prepared in an identical manner to the blue sensitiser, except that the dye used is Rose Bengal (CI 45,440), and 60 mg are dissolved in 40 ml methanol.

Red Sensitiser.

The red sensitiser is prepared in an identical manner to the blue sensitiser, except that the dye used is Patent Blue V (CI 42,045).

60 mg are dissolved in 40 ml methanol, and the final formulation for ball-milling is as follows 2 g dye sensitised zinc oxide 2 g soya modified alkyd resin 20 ml odourless paraffin C. sensitisation of Zinc Oxide Paper to produce Photoconductive Material of the Mosaic type.

The'unsensitised zinc oxide paper is charged by passing it under a corona wire held at 10KV negative. The same corona wire is used for all charging operations in this example, and normally gives a charge acceptance of 200V. The paper is then exposed through a line screen to a watt tungsten lamp held at a distance of 30 cm inches for 0.25 see. This lamp (with or without coloured filters) is used for all the exposures in the example. The line screen has black lines of half the width of the interspaces and has a spacing of lines/cm. The charge remaining is toned by holding the paper in the blue sensitiser for 2 sec. and drying in a stream of warm air.

The paper is recharged, exposed uniformly to cyan light (Ilford filters 303, 805 and 809 in combination) for 0.5 sec. and then exposed to a line screen held at right angles to the direction of the first screen using unfiltered light for 0.25 sec. This screen also has a spacing of 120 lines/cm, but the line width is equal to the interspace. The charge remaining is toned by holding the paper in the green sensitiser for 2 sec. and drying a stream of warm air.

Thepaper is again recharged and exposed uniformly to cyan light (Ilford filters 303, 805 and 809 in combination) for 1.5 sec. The charge remaining is toned by holding the paper. in the red sensitiser for 2 sec. and drying in a stream of warm air.

The paper has now been sensitised with a mosaic of blue, green and red sensitive areas, and is ready for use. We claim as our invention 1. A process for the production of colour photoconductive material of the mosaic type, wherein areas of the material are sensitive only to selected regions of the visible spectrum, from spectrally unsensitised photoconductive material, which comprises either a photoconductor layer coated on an electroconductive layer which is in its turn coated on a support base or a photoconductive layer coated on a support base which is or has been rendered electroconductive, said process comprising the steps of a. charging the surface of the photoconductive material,

b. exposing the surface of the material to a pattern of light of such wavelength to which the unsensitised photo-conductor layer is sensitive,

c. toning the areas remaining charged on the surface of the photoconductive material by means of a dye sensitiser which sensitises them to light of one of the selected regions of the spectrum;

followed by at least one further sequence of the steps d, e, f and g which comprise,

d. recharging the surface of the photoconductive material,

e, exposing the surface overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitised thereby to destroy the charge on the material selectively in t such areas,

prior to, or at the same time as, or after this overall light exposure, exposing the surface of the material to another pattern of light of such wavelength to which the unsensitised photoconductor layer is sensitive the said pattern being different from any pattern of light to which the material has been exposed previously,

toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitiser which sensitises them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive, and then h. recharging the surface of the photoconductive material,

i. exposing the surface of the material overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitised thereby to destroy the charge on the material selectively in such areas,

j. toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitiser which sensitises them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive.

2. A process according to claim 1 for producing mosaic material sensitive to three selected regions of the spectrum which comprises carrying out the steps a, b and c, followed by one sequence of the steps d, e, f and g, followed by the steps h, i and j.

3. A process according to claim 2 which comprises using two patterns of light to which the photoconductive material is exposed in a first exposure b and in a second exposure e which are such that the areas of the final colour photo-conductive material which are sensitive to only one of the three regions of the spectrum are equal.

4. A process for the production of colour photo conductive material of the mosaic type, wherein areas of the material are sensitive only to selected regions of the visible spectrum, from spectrally unsensitised photo-conductive material, which comprises either a photo-conductor layer coated on an electrodonductive layer which is in its turn coated on a support base or a photo-conductive layer coated on a support base which is or has been rendered electroconductive, which comprises the steps of a. charging the surface of the photoconductive material,

b. exposing the surface of the material to light of such wavelength to which the unsensitised photoconductor layer is sensitive through a screen which comprises a series of lines which are opaque to the said light.

c. toning the areas remaining charged on the surface of the photoconductive material by means of a dye sensitiser which sensitises them to light of one of the selected regions of the spectrum;

d. recharging the surface of the photoconductive material,

e. exposing the surface overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitised thereby to destroy the charge on the material selectively in such areas,

f. prior to, or at the same time as, or after this overall light exposure, exposing the surface of the material to light of such wavelength to which the unsensitised photo-conductor layer is sensitive through a screen which comprises a series of lines which are opaque to the said light, the lines of the screen in this exposure crossing at an angle to the direction of the lines during the exposure is carried out in b, toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitiser which sensitises them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive, and then h. recharging the surface of the photoconductive material i. exposing the surface of the material overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitised thereby to destroy the charge on the material selectively in such areas,

j. toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitiser which sensitises them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive.

5. A process according to claim 4 which comprises using in the first screen exposure of stage b, a screen in which the area occupied by the said opaque lines is a third of the area of the screen.

6. A process according to claim 5 which comprises using a screen wherein each opaque line is of the same width.

7. A process according to claim 4 which comprises using in the first screen exposure a screen comprising a, series of lines which are opaque to all light.

8. A process according to claim 4 which comprises using in the second screen exposure of stage f a screen ing a series of lines which are opaque to light of that region of the spectrum to which the unsensitised photoconductor layer is sensitive but which transmit light in that region of the spectrum to which the areas of the photoconductive layer toned in step c are sensitive.

12. A process according to claim 4 which comprises using in the second screen exposure of stage f a screen wherein the lines cross the direction of the lines in the first screen exposure of stage b at right angles. 

2. A process according to claim 1 for producing mosaic material sensitive to three selected regions of the spectrum which comprises carrying out the steps a, b and c, followed by one sequence of the steps d, e, f and g, followed by the steps h, i and j.
 3. A process according to claim 2 which comprises using two patterns of light to which the photoconductive material is exposed in a first exposure b and in a second exposure e which are such that the areas of the final colour photo-conductive material which are sensitive to only one of the three regions of the spectrum are equal.
 4. A process for the production of colour photo-conductive material of the mosaic type, wherein areas of the material are sensitive only to selected regions of the visible spectrum, from spectrally unsensitised photo-conductive material, which comprises either a photo-conductor layer coated on an electrodonductive layer which is in its turn coated on a support base or a photo-conductive layer coated on a support base which is or has been rendered electroconductive, which comprises the steps of a. charging the surface of the photoconductive material, b. exposing the surface of the material to light of such wavelength to which the unsensitised photoconductor layer is sensitive through a screen which comprises a series of lines which are opaque to the said light. c. toning the areas remaining charged on the surface of the photoconductive material by means of a dye sensitiser which sensitises them to light of one of the selected regions of the spectrum; d. recharging the surface of the photoconductive material, e. exposing the surface overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitised thereby to destroy the charge on the material selectively in such areas, f. prior to, or at the same time as, or after this overall light exposure, exposing the surface of the material to light of such wavelength to which the unsensitised photo-conductor layer is sensitive through a screen which comprises a series of lines which are opaque to the said light, the lines of the screen in this exposure crossing at an angle to the direction of the lines during the exposure is carried out in b, g. toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitiser which sensitises them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive, and then h. recharging the surface of the photoconductive material i. exposing the surface of the material overall to light which comprises all the regions of the visible spectrum to which areas thereof have been sensitised thereby to destroy the charge on the material selectively in such areas, j. toning the areas remaining charged on the surface of the photoconductor by means of a dye sensitiser which sensitises them to light of a selected region of the spectrum to which no area of the surface of the material is at present sensitive.
 5. A process according to claim 4 which comprises using in the first screen exposure of stage b, a screen in which the area occupied by the said opaque lines is a third of the area of the screen.
 6. A process according to claim 5 which comprises using a screen wherein each opaque line is of the same width.
 7. A process according to claim 4 which comprises using in the first screen exposure a screen comprising a series of lines which are opaque to all light.
 8. A process according to claim 4 which comprises using in the second screen exposure of stage f a screen whereby the area occupied by the said opaque lines is one half of the area of the screen.
 9. A process according to claim 8 which comprises using a screen wherein each opaque line is of the same width.
 10. A process according to claim 4 which comprises using in the second screen exposure of stage f a screen comprising a series of lines which are opaque to all light.
 11. A process according to claim 4 which comprises using in the second screen exposure a screen comprising a series of lines which are opaque to light of that region of the spectrum to which the unsensitised photoconductor layer is sensitive but which transmit light in that region of the spectrum to which the areas of the photoconductive layer toned in step c are sensitive.
 12. A process according to claim 4 which comprises using in the second screen exposure of stage f a screen wherein the lines cross the direction of the lines in the first screen exposure of stage b at right angles. 